The following abbreviations are herewith defined:    3GPP third generation partnership project    ACK acknowledgement    AICH acquisition indicator channel    ARQ automatic repeat request    BCH broadcast channel    CPCH common packet channel    E-AGCH E-DCH absolute grant channel (in downlink)    E-DCH enhanced dedicated channel    E-DPCCH enhanced dedicated physical control channel (uplink L1 information concerning data rate)    E-DPDCH enhanced dedicated physical channel (uplink data channels)    E-HICH E-DCH hybrid ARQ indicator channel (in downlink)    E-RGCH E-DCH relative grant channel    E-TFC E-DCH transport format combination    FACH forward access channel    F-DPCH fractional dedicated physical channel    HARQ hybrid automatic repeat request    HS high speed    HSUPA high speed uplink packet access    L1 Layer 1 (physical)    MAC medium access control    NACK negative acknowledgement    Node B base station    PC power control    RACH random access channel    RNC radio network controller    RRC radio resource control    SIB system information block    TCP transport control protocol    UE user equipment, such as a mobile station or mobile terminal    WCDMA wideband code division multiple access
The data rates for RACH (Release 99) are very low. More specifically, the practical instantaneous data rate is 16 kbps (16 thousand bits per second, or 320 bits in 20 ms) and effectively <10 kbps when preamble power ramping is taken into account. The RACH thus cannot be used in any practical manner for the transmission of large packet calls, and no enhancements to RACH were proposed in later releases (post Release 99). Further, in the event of a failed RACH transmission the retransmission delays are significant, on the order of seconds.
In addition, in Release 99 the dedicated channel setup times from CELL_FACH to CELL_DCH are substantial, which is partially due to the signaling needed before the dedicated connection can be established using the slow RACH. This leads to considerably longer delays than are necessary for small to medium sized amounts of data to be transferred. A large part of the total time is spent before any bits can actually be delivered in the CELL_DCH state. Using the HSUPA physical channel that is currently only available in the CELL_DCH state increases the data rates considerably in comparison to what the RACH can provide. In such a manner, it is possible to start data transmission in the CELL_FACH state using the HSUPA channels with practically no break in the transition to the CELL_DCH state, where the transmission can continue using the same channel.
The bit rate of the RACH is set, and can be changed by RRC signaling. However, this can be a burdensome process. Furthermore, the RRC signaling itself may cause a state change in the CELL_DCH.
A HS-FACH concept has been proposed in 3GPP as R2-061189, “Further discussion on delay enhancements in Re17”, Nokia, 8-12 May 2006. The proposed HS-FACH can provide downlink CELL_FACH state data rates of hundreds of kbps to greater than 1 Mbps (million bits per second). However, in order to obtain the full benefit of the improved downlink, corresponding CELL_FACH state capability improvements would be needed in the uplink CELL_FACH state capabilities. Especially for TCP-based applications, the end user experience will depend as well on the uplink data rates if the downlink data rates are significantly increased, since TCP acknowledgements are sent in the reverse link.
A CPCH was included in 3GPP Release 99 specifications and in some subsequent releases as well. The CPCH was intended to be an extension of RACH. Reference in this regard can be made to:
3GPP TS25.211, V4.6.0 (2002-September), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 4);
3GPP TS25.212, V4.6.0 (2002-September), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD) (Release 4);
3GPP TS25.213, V4.4.0 (2003-December), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Spreading and modulation (FDD) (Release 4);
3GPP TS25.214, V4.6.0 (2003-March), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 4); and
3GPP TS25.215, V4.8.0 (2005-March), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer-Measurements (FDD) (Release 4).
The CPCH was defined in Section 4.1.2.5 of 3GPP TS25.211 V4.6.0 as being an uplink transport channel that is associated with a dedicated channel on the downlink which provides power control and CPCH control commands for the uplink CPCH. The CPCH is characterized by having an initial collision risk, and by being transmitted using inner loop power control commands.
However, the CPCH was not implemented and it was removed from 3GPP Release 5 specifications. The CPCH thus did not include L1 enhancements, since these enhancements were included for the uplink only with HSUPA in Release 6. The L1 enhancements include fast L1 retransmission, Hybrid ARQ, and fast capacity allocations.
Notably, the allocation of a bit rate on the CPCH was fixed, as on the RACH.